Field of the Invention
The present invention relates to an image-processing apparatus and an image-processing method.
Description of the Related Art
In recent years, there has been a growing use of image data having a wide dynamic range. For example, with the improvement in light-receiving performance of imaging apparatuses, imaging apparatuses capable of generating image data having a wide dynamic range are now being used. Image data with a wide dynamic range is referred to as “high-dynamic-range (HDR) image data.” As a data format for HDR image data, data formats having gradation characteristics (log characteristics) such that the gradation value increases logarithmically with respect to an increase in brightness have been proposed. For example, at movie production sites, Cineon log that has been specified based on the characteristics of a film with a wide dynamic range has been used as a data format.
With the improvement in display performance of image display apparatuses, display at a high contrast of several hundred thousand to one, display at a brightness of several thousand candelas, and the like are now performed. For example, in liquid crystal display apparatuses, display of high contrast can be realized by local dimming control using a backlight that illuminates a liquid crystal panel. By increasing the brightness of light emitted by the backlight, high-brightness display can be realized.
Studies are made on an image production workflow for displaying an image based on HDR image data in an image display apparatus capable of realizing high-contrast and high-brightness display. Conventionally, Rec. 709 and the like have been used as a transmission standard for transmitting image data to an image display apparatus. However, with conventional transmission standards, the dynamic range was limited to a narrow range, and the brightness was limited, to low brightness. Therefore, conventionally, image data was converted to image data for Rec. 709 by image editing involving range compression to narrow the dynamic range, and the image data after conversion was transmitted to an image display apparatus. Then, in the image display apparatus, an image was displayed using only part of the capabilities of the image display apparatus. That is, due to the limitations of the transmission standards, there have been losses in the dynamic range of image data.
In this regard, studies are being made on a transmission scheme to maintain the dynamic range of HDR image data. In the transmission scheme to maintain the dynamic range of HDR image data, conversion such as that shown in FIGS. 10 and 11 is performed. First, as shown in FIG. 10, each gradation value of HDR image data is converted to brightness “cd”. The conversion characteristics for converting the gradation value to the brightness are not limited to the conversion characteristics shown in FIG. 10. For example, in a case of editing an image, an image producer decides on the brightness for display of each gradation value, and, based on the result of decision, determines the conversion characteristics for converting the gradation value to the brightness. Next, as shown in FIG. 11, each brightness of HDR image data is converted to the gradation value. The conversion characteristics for converting the brightness to the gradation value are not limited to the conversion characteristics shown in FIG. 11. For example, the conversion characteristics for converting the brightness to the gradation value are determined based on the visual ability to discriminate brightness.
In an image display apparatus, unevenness (display unevenness) in at least one of brightness and color occurs in a screen in a case of displaying an image in the screen. Therefore, in the image display apparatus, unevenness reduction processing of reducing the display unevenness is performed. For example, brightness unevenness is measured in advance using a measuring instrument. As shown in FIG. 12, a plurality of divided areas forming a screen area are specified in advance. Based on the result of measurement, processing of reducing the brightness of a divided area where the brightness is high to the brightness of a divided area where the brightness is low is performed as the unevenness reduction processing.
However, general unevenness reduction processing causes a decrease in display brightness (brightness on the screen). For example, in the unevenness reduction processing described above, the display brightness decreases since the brightness of a divided area where the brightness is high is reduced to the brightness of a divided area where the brightness is low. Therefore, the unevenness reduction processing limits the upper limit value of display brightness to a low value and generates a gradation value (gradation range) with which the brightness of image data cannot be reproduced accurately.
Thus, a technique is proposed in which the upper limit value of display brightness is maintained by reducing the degree of reduction in display unevenness by the unevenness reduction processing to a smaller value for a higher gradation value (for example, see Japanese Patent Application Laid-open No. 2008-310261).